In recent years, as miniaturization, light weight and high performance of electronic equipment have been required, there has been a strong demand not only for a small size and light weight of a circuit board but also for higher speed for processing signals and high density mounting, and the like. In order to meet such demands, in the field of a circuit board technology, a technology for increasing the number of layers in a laminate structure, a technology of making the diameter of an inner via hole smaller, a technology for making circuit patterns finer, and the like have been developed rapidly, and circuit boards having various structures have been proposed or put into practice. As one example, the present assignee has developed a circuit board having an all-layer IVH (interstitial inner via hole) structure, in which the interlayer electric connection is secured with an inner via hole filled with a conductive paste in place of a through hole structure that has been a usual method in conventional circuit boards (see, for example, JP7 (1995)-147464 A). This circuit board has features that no through hole is needed and inner via holes can be arranged at arbitrary positions, and therefore, this circuit board has a high capacity of wiring, a high degree of freedom in design, shortening of wiring, etc. Thus, this circuit board is suitable for signal processing at high speed and high density mounting. In this circuit board, the conductive paste filled in the through hole is compressed and hardened simultaneously with molding of the laminate, thereby providing an inner via hole for securing an interlayer electrical connection. Therefore, this circuit board needs an electrically insulating material having such a compressive performance that permits sufficient compression of a conductive paste and is capable of maintaining the shape of the inner via hole. In general, as a conductive paste, a prepreg obtained by impregnating an aramid non-woven fabric with a thermosetting resin is used. The compressive performance of the aramid prepreg can be achieved by the melt flow of the thermosetting resin and the effect of defoaming of any remaining air foam. The maintenance of the shape of the inner via hole can be achieved by rigid property of the aramid non-woven fabric.
In the future, in a view that a circuit board will increasingly be required to have a finer circuit pattern, a smaller diameter of an inner via hole and an increase in number of the layers of the laminate structure and the like, it is extremely important to make the electrically insulating layer to be homogenized and ultra-thin. However, a non-woven fabric made of an aramid staple fiber having a diameter of 10–20 μm may cause a non-negligible non-uniformity of the physical property in a micro area depending on the direction of the fiber. Furthermore, a satisfactory thin layer cannot be obtained because there is a limitation to the number of fibers in the thickness direction.